Conventionally, a high-output battery pack includes a switch between an internal battery and external load for safety's sake, and a main circuit is disconnected from the external load by opening the switch when the battery pack is not in use.
When using the battery pack, the internal battery of the battery pack can be connected to the external load by closing the switch. However, if a voltage difference (to be referred to as “an internal-external voltage difference” hereinafter) between the internal battery and external load is large, a large electric current flows when they are connected, so a component of the switch or the like is sometimes broken.
Accordingly, the following method is known as an example of a control method of suppressing this large electric current when connecting the battery and load. That is, this method measures the internal-external voltage difference, and, if the measured internal-external voltage difference is larger than a predetermined value, connects the external load via an output-limiting resistance for limiting an electric current to be output to the external load. The control method closes the switch if the internal-external voltage difference becomes smaller than the predetermined value.
This conventional control method will briefly be explained with reference to FIG. 1. FIG. 1 shows a circuit example of a conventional battery pack 60 and an external load unit 80 connected to the battery pack 60. Note that in addition to the circuit as shown in FIG. 1, the battery pack 60 includes a controller (not shown) for controlling elements forming the circuit.
The battery pack 60 is a battery pack such as a lithium-ion battery. The battery pack 60 includes an internal battery unit 61, switch SW0, internal voltage measurement unit 65, external voltage measurement unit 66, current measurement unit 67, positive terminal 68, negative terminal 69, and output-limiting resistance unit 70.
Also, the internal battery unit 61 includes an internal battery 62 and internal battery resistance 63. The output-limiting resistance unit 70 includes an output-limiting resistor 72 having a resistance value R01, and an output-limiting resistance switch SW1.
On the other hand, the external load unit 80 to be connected to the battery pack 60 includes a battery load unit 82 and resistance/coil/capacitance load unit 83. The battery load unit 82 includes an external battery 85 and external battery resistor 84. The resistance/coil/capacitance load unit 83 includes a resistance/coil/capacitance load 86 which is one of a resistance load, coil load, and capacitance load. For example, the coil load is that of a starter installed in a vehicle.
In this arrangement, the external load unit 80 includes the battery load unit 82, and this produces a voltage difference (V1−V) (to be referred to as “an internal-external voltage difference” hereinafter) between an internal voltage V1 measured by the internal voltage measurement unit 65 of the battery load unit 82, and an external voltage V measured by the external voltage measurement unit 66.
This internal-external voltage difference disappears with the elapse of time as shown in FIG. 2. FIG. 2 is an example of a graph schematically showing changes in internal voltage V1 and external voltage V with time, when the output-limiting resistance switch SW1 is closed with the battery load unit 82 being included in the external load unit 80.
Initially, the internal voltage V1 is 12 V, and the external voltage V is 8 V, so there is an internal-external voltage difference. Then, the internal-external voltage difference gradually decreases because an electric current I1 measured by the current measurement unit 67 flows through the output-limiting resistance unit 70. When a few ten hours (e.g., ten hours) elapse, the internal-external voltage difference disappears, and the battery pack 60 and external load unit 80 are completely connected by closing the switch SW0.
Unfortunately, even when using the conventional control method as described above, a large electric current may flow when the battery pack 60 is connected to the external load unit 80 if the internal-external voltage difference is large.
Also, when the external load is a battery load, coil load, or capacitance load, the internal-external voltage difference can be eliminated through the output-limiting resistor as described above. However, when an external load unit 90 includes a resistance load unit 92 or constant-current load unit 93 as shown in FIG. 3, the internal-external voltage difference keeps taking a predetermined value as shown in FIGS. 4 and 5. Accordingly, the internal-external voltage difference does not disappear but takes the predetermined value, so the switch SW0 cannot be closed even after the elapse of time.
Assume that, as shown in FIG. 3, the external load unit 90 includes the resistance load unit 92 including a resistor 94 and a constant-current load 93 including a constant-current source 95. Assume also a conventional control method using only the internal-external voltage difference as a closing determination condition for determining whether to close the switch SW0. In this case, even when the switch SW0 can safely be closed, it is determined that the switch SW0 cannot be closed because the internal-external voltage difference keeps taking the predetermined value.
FIG. 4 is an example of a graph schematically showing changes in internal voltage V1 and external voltage V with time, when the external load unit 90 includes the resistance load unit 92, and the output-limiting resistance switch SW1 is closed.
As shown in FIG. 4, the internal voltage V1 is 10 V, the external voltage V is about 6.5 V, and the internal voltage V1 and external voltage V keep taking the predetermined values even when time elapses. Since the internal-external voltage difference does not disappear, therefore, the switch SW0 cannot be closed, so the resistance load cannot be connected.
Likewise, FIG. 5 is an example of a graph schematically showing changes in internal voltage V1 and external voltage V with time, when the external load unit 90 includes the constant-current load unit 93, and the output-limiting resistance switch SW1 is closed.
As shown in FIG. 5, the internal voltage V1 is 10 V, the external voltage V is about 9.5 V, and the internal voltage V1 and external voltage V keep taking the predetermined values even when time elapses. Since the internal-external voltage difference does not disappear, therefore, the switch SW0 cannot be closed, so the constant-current load cannot be connected.
From the foregoing, it is desirable to provide a battery pack and control method for safely and reliably connecting an external load.